i86 



ZOOLOGY. 



pressed in a dorso-ventral direction, and from this fact the name is given. 

 They are soft-bodied animals without any true skeleton. There is no 

 body cavity and no true blood-vascular system. The space which would 

 be given to such structures is filled with a spongy connective tissue. 



FIG. 86. 



FIG. 86. Diagram ot a Turbellarian, showing the general arrangement of the nervous 

 structures and one of the modes of occurrence of the excretory tubules, which in this 

 case open separately into the pharynx, on the ventral side of the animal, b., brain; e, 

 eye-spots; ex, excretory canals consisting of a transverse portion passing from the 

 mouth toward the dorsal side (see also Fig. 87), and longitudinal tubes which branch 

 into the capillary vessels terminating in f, the flame cells; lc., lateral nerve cords; m, 

 mouth. 



Questions on the figure. Compare this figure with the next and 

 identify the structures shown in both. What other positions of the mouth 

 do you discover in the Turbellaria, as figured in reference texts? What 

 other arrangement of the excretory canals and pores ? 



Through this body-mass run the minute tubes of the excretory or water- 

 vascular system (Fig. 86, ex.}, often terminating internally in special cells 

 (flame cells, Fig. 88). These tubes have external pores. By means 

 of this system of organs waste products, probably of a nitrogenous nature, 

 are eliminated from the tissues. The digestive tract may be wholly want- 

 ing as in the Cestodes, or a simple or forked sac, or a central sac with 

 lateral branches. It is blind, *. e., has only the oral opening. In the more 

 complicated types of stomach the much-branched sac serves the function 

 of carrying the digested food to all parts of the body. Many of these 

 forms are parasitic and in consequence the organs referred to are often 



